Process and catalyst for producing syndiotactic polymers

ABSTRACT

Syndiospecific catalysts and processes for the syndiotactic propagation of a polymer chain derived from an ethylenically unsaturated monomer which contains 3 or more carbon atoms or is a subsituted vinyl compound. The catalysts comprise an unbalanced metallocene cation having sterically dissimilar ring structures joined to a positively charged coordinating transition metal atom and a stable noncoordinating counter anion for the metallocene cation. Both ring structures are substituted cyclopentadienyl rings and one is sterically different from the other. Both are in stereorigid relationship to the coordinating transition metal atom to prevent rotation of the rings by direct stearic hindrence between the ring structures. The catalyst is contacted with a C3+ alpha olefin or other ethylenically unsaturated compound in a polymerization reaction zone and maintained in contact with the catalyst in the reaction zone under polymerization conditions to produce a syndiotactic polymer.

This application is a continuation of application Ser. No. 418,497,filed Oct. 10, 1989 and now abandoned which, in turn, is acontinuation-in-part of application Ser. No. 220,007, filed Jul. 15,1988 and now U.S. Pat. No. 4,892,851.

TECHNICAL FIELD

This invention relates to catalysts and processes for the production ofsyndiotactic polymers from ethylenically unsaturated compounds and moreparticularly to the production of a syndiotactic polyolefin bypolymerization of propylene or higher alpha olefin over a stereorigidcationic metallocene catalyst having dissimilar cyclopentadienyl rings.

BACKGROUND OF THE INVENTION

Syndiotacticity is one of a number of stereospecific structuralrelationships which may be involved in the formation of the stereorigidpolymers which may be derived from various monomers. Stereo-specificpropagation may be applied in the polymeri-zation of ethylenicallyunsaturated monomers such as C3+alpha-olefins, 1-dienes such as1,3-butadiene and additional or substituted vinyl compounds such asvinyl aromatics, e.g., styrene or vinyl chloride, vinyl ethers such asalkyl vinyl ethers, e.g., isobutyl vinyl ether, or even aryl vinylethers. Stereospecific polymer propagation is probably of mostsignificance in the production of polypropylene of isotactic orsyndiotactic structure.

Syndiotactic polymers have a unique stereochemical structure in whichmonomeric units having enantiomorphic configuration of the asymmetricalcarbon atoms follow each other alternately and regularly in the mainpolymer chain. Syndiotactic polypropylene was first disclosed by Nattaet al. in U.S. Pat. No. 3,258,455. As disclosed in this patent,syndiotactic polypropylene can be produced by using a catalyst preparedfrom titanium trichloride and diethyl aluminum monochloride. A laterpatent to Natta et al., U.S. Pat. No. 3,305,538, discloses the use ofvanadium triacetylacetonate or halogenated vanadium compounds incombination with organic aluminum compounds for producing syndiotacticpolypropylene. U.S. Pat. No. 3,364,190 to Emrick discloses the use of acatalyst system composed of finely divided titanium or vanadiumtrichloride, aluminum chloride, a trialkyl aluminum and aphosphorus-containing Lewis base in the production of syndiotacticpolypropylene.

As disclosed in these patent references and as known in the art, thestructure and properties of syndiotactic polypropylene differsignificantly from those of isotactic polypropylene. The isotacticstructure is typically described as having the methyl groups attached tothe tertiary carbon atoms of successive monomeric units on the same sideof a hypothetical plane through the main chain of the polymer, e.g., themethyl groups are all above or below the plane. Using the Fischerprojection formula, the stereochemical sequence of isotacticpolypropylene is described as follows: ##STR1##

Another way of describing the structure is through the use of NMR.Bovey's NMR nomenclature for an isotactic pentad is . . . mmmm . . .with each "m" representing a "meso" dyad or successive methyl groups onthe same side in the plane. As known in the art, any deviation orinversion in the structure of the chain lowers the degree ofisotacticity and crystallinity of the polymer.

In contrast to the isotactic structure, syndiotactic polymers are thosein which the methyl groups attached to the tertiary carbon atoms ofsuccessive monomeric units in the chain lie on alternate sides of theplane of the polymer. Syndiotactic polypropylene shown in zig-zagrepresentation as follows: ##STR2##

Corresponding representations for syndiotactic polyvinylchloride andpolystyrene respectively are: ##STR3## Using the Fischer projectionformula, the structure of a syndiotactic polymer or polymer block forpolypropylene is designated as: ##STR4## In NMR nomenclature, thispentad is described as . . . rrrr . . . in which each "r" represents a"racemic" dyad, i.e., successive methyl groups on alternate sides of theplane. The percentage of r dyads in the chain determines the degree ofsyndiotacticity of the polymer.

Syndiotactic polymers are crystalline and, like the isotactic polymers,are insoluble in xylene. This crystallinity distinguishes bothsyndiotactic and isotactic polymers from an atactic polymer that issoluble in xylene. An atactic polymer exhibits no regular order ofrepeating unit configurations in the polymer chain and forms essentiallya waxy product.

While it is possible for a catalyst to produce all three types ofpolymers, it is desirable for a catalyst to produce predominantlyisotactic or syndiotactic polymer with very little atactic polymer.Catalysts that produce isotactic polyolefins are disclosed in copendingU.S. patent application Ser. Nos. 034,472 filed Apr. 3, 1987; 096,075filed Sep. 11, 1987 and now U.S. Pat. No. 4,794,096; and 095,755 filedon Sep. 11, 1987. These applications disclose chiral, stereorigidmetallocene catalysts that polymerize olefins to form isotactic polymersand are especially useful in the polymerization of a highly isotacticpolypropylene.

Catalysts that produce syndiotactic polypropylene or other syndiotacticpolyolefins are disclosed in the aforementioned U.S. Pat. No. 4,892,851.These catalysts are bridged stereorigid metallocene catalysts.

The catalysts have a structural bridge extending between dissimilarcyclopentadienyl groups and may be characterized by the formula:

    R"(CpRn)(CpR'm)MeQk                                        (1)

In formula (1), Cp represents a cyclopentadienyl or substitutedcyclopentadienyl ring; and R and R' represent hydrocarbyl radicalshaving 1-20 carbon atoms.

R" is a structural bridge between the rings imparting stereorigidity tothe catalyst; Me represents a transition metal and Q a hydrocarbylradical or halogen.

R'm is selected so that (CpR'm) is a sterically different substitutedcyclopentadienyl ring than (CpRn); n varies from 0 to 4 (0 designatingno hydrocarbyl groups, i.e. an unsubstituted cyclopentadienyl ring) andm varies from 1-4, and K is from 0-3. The sterically differentcyclopentadienyl rings produces a predominantly syndiotactic polymerrather than an isotactic polymer.

Metallocene catalysts of yet another type are cationic catalysts asdisclosed in European Patent Applications 277,003 to Turner et al. and277,004 to Turner. As disclosed in these applications, abis(cyclopentadienyl) zirconium, titanium or hafnium compound is reactedwith a second compound comprising a cation capable of donating a protonor an ion exchange compound comprising a cation which will irreversiblereact with a ligand on the first compound, and a bulky, stable anion.The catalysts described in the European Patent Applications 277,003 and277,004 are disclosed as especially useful in the polymerization ofethylene and more generally in the polymerization of alpha olefins,diolefins and/or an acetylenically unsaturated compounds containing from2-18 carbon atoms. Principally disclosed in the European applications isthe polymerization of ethylene or the copolymerization of ethylene withpropylene or 1-butene or with propylene and 1-butene or 1,4 hexadiene.Stereospecificity, or lack thereof, of the polymers as disclosed in theTurner and Turner et al. applications is not generally discussed,although in Application 277,004 examples are given of producing atacticpolypropylene and in one instance (Example 39) isotactic polypropylene.

SUMMARY OF THE INVENTION

In accordance with the present invention, there are providedsyndiospecific catalysts and processes for the syndiotactic propagationof a polymer chain derived from an ethylenically unsaturated monomerwhich contains 3 or more carbon atoms or is a substituted vinylcompound. Catalysts in accordance with the present invention comprise anunbalanced metallocene cation and a stable noncoordinating counter anionfor the metallocene cation. The metallocene cation is characterized by acationic metallocene ligand having sterically dissimilar ring structuresjoined to a positively charged coordinating transition metal atom. Bothring structures are substituted cyclopentadienyl rings and one of thering structures is a substituted cyclopentadienyl group which issterically different from the other substituted cyclopentadienyl group.Both of said cyclopentadienyl groups are in a stereorigid relationshiprelative to the coordinating transition metal atom to prevent rotationof said rings because of direct stearic hindrance between the ringstructures.

Syndiotactic polypropylene or other polymers resulting from thepolymerization of C3+ alpha olefins or vinyl compounds may be producedin accordance with the method of the present invention. Syndiospecificpropagation of the polymer chain is carried out in the presence of astereorigid cationic metallocene catalyst which incorporates dissimilarsubstituted cyclopentadienyl rings in direct stearic hindrance with oneanother so that both are in a stereorigid relationship relative to thecoordinating metal atom of the metallocene complex. The catalyst iscontacted with a C3+ alpha olefin or other ethylenically unsaturatedcompound in a polymerization reaction zone and maintained in contactwith the catalyst in the reaction zone under polymerization conditionsto produce a syndiotactic polymer. The preferred application of theinvention is in the production of syndiotactic polypropylene.

Specific catalysts employed in which the aforementioned stereorigidrelationship between the dissimilar cyclopentadienyl rings isestablished in accordance with the present invention may becharacterized by the following formula:

    [(CpSx)(CpS'y) MeQk]+.sub.a -                              (2)

wherein:

Cp is a cyclopentadienyl or a substituted cyclopentadienyl ring;

each S is the same or different and is a hydrocarbyl radical having from1-20 carbon atoms;

each S' is the same or different and is a hydrocarbyl radical havingfrom 1-20 carbon atoms and selected such that CpSx is a stericallydifferent ring than CpSy and is in a sterically hindered relationshiprelative to catalyst;

Me is a Group 4, 5, or 6 metal from the Periodic Table of Element;

Q is a hydrocarhyl radical having from 1-20 carbon atoms or is ahalogen;

x is from 1 to 5; y is from 1 to 5; k is from 0 to 2; and

_(a) is a stable noncoordinating counter ion.

DETAILED DESCRIPTION

The present invention involves certain stereorigid cationic metallocenesand their use as catalysts in syndiotactic polymer propagation. The termmetallocene as used herein and in accordance with normal art usagedenotes an organometallic coordination compound in which two cyclo-C5ligands (cyclopentadienyl or substituted cyclopentadienyl rings) arebonded to a central or "sandwiched" metal atom which may be provided bya transition metal or metal halide, alkyl, alkoxy, or alkyl or alkoxyhalide or the like. Such structures are sometimes referred to as"molecular sandwiches" since the cyclo-C5 ligands are oriented above orbelow the plane of the central coordinated metal atom. By the term"cationic metallocene" is meant a metallocene in which the centralcoordinated metal atom carries a positive charge, that is, themetallocene complex is a cation associated with a stable anion. Thecationic metallocenes involved in the present invention are stereorigid.Stereorigidity is imparted to the metallocene complex to preventrotation of substituted cyclopentadienyl rings about their coordinationaxes by direct stearic hindrance. That is stereorigidity is imposed bymeans of substituted cyclopentadienyl rings in which the substituentgroups provide for stearic hindrance in the conventional sense ofnonbonded spacial interaction between the two substitutedcyclopentadienyl rings.

As noted previously, U.S. Pat. No. 4,892,851 discloses the preparationof syndiotactic polypropylene, or other polyolefins, through the use ofstereorigid metallocene catalysts. The present invention employsstereorigid metallocene catalysts similar in some respects to thosedisclosed in U.S. Pat. No. 4,892,851 in the sense that a metalloceneligand is ionized to provide a stable cationic catalyst. The cationicmetallocene catalysts employed in the present invention may be preparedfollowing procedures of the type disclosed in the aforementionedEuropean Applications 277,003 and 277,004, but they preferably areprepared by a process employing a triphenylcarbenium borate as discussedin greater detail below. Where procedures of the type disclosed in theEuropean applications are used in the preparation of cationicmetallocene catalysts to be employed in the present invention, certainimportant distinctions must be observed as evidenced by the fact thatneither of the European applications disclose the preparation ofsyndiotactic polymers. Thus, in the metallocene catalysts disclosed inthe European applications, the cyclopentadienyl groups may be the sameor different, and while they can be bridged, they need not be and, infact, are usually unbridged. Moreover, to the extent that themetallocene catalysts disclosed in the European applications are bridgedto impart stereorigidity, they are also symmetrical. In contrast to theteachings of the Turner European applications, the cationic metallocenecatalysts employed in the present invention must not only be stereorigidwith stereorigid provided by stearic hindrance, the cyclopentadienylgroups must be dissimilar.

Stereorigid cationic metallocene catalysts employed in the presentinvention may be characterized by the following formula:

    [(CpSx)(CpS'y) MeQk]+P.sub.a -                             (2)

wherein: Cp, R, R', R", S, S', T, T', Me, Q, P_(a), k, m, n, x and y areas described previously.

In the catalysts of formula (2), stereorigidity is imparted by means ofdirect stearic hindrance between the two substituted cyclopentadienylgroups provided by relatively bulky or long chain substituents groupsrepresented by S and S'.

The counter anion indicated by P in formula (2) is a compatiblenoncoordinating anion which may be of the type described in theaforementioned Turner European applications. The anion P either does notcoordinate with the metallocene cation or is only weakly coordinated tothe cation thereby remaining sufficiently labile to be displaced by aneutral Lewis base. As described in the Turner applications, the term"compatible noncoordinating anion" identifies an anion which whenfunctioning as a stabilizing anion in the metallocene catalyst systemdoes not transfer an anionic substituent or fragment thereof to thecation to form a neutral metallocene and boron byproduct or otherneutral metal or metalloid byproduct, as the case may be. Suitablenoncoordinating anions include: [W(PhF5)]-, [Mo(PhF5)-] (wherein PhF5 ispentafluoryl phenol) [C104]-, [PF6]-, [SbR6]-, [AlR4] (wherein each R isindependently, C1, a C1-C5-alkyl group, preferably a methyl group, anaryl group, e.g., a phenyl or substituted phenyl group, or a fluorinatedaryl group. For a further description of compatible noncoordinatinganions and their associated cations which may be employed in the presentinvention, reference is made to European applications 277,003 and277,004, the entire disclosures of which are incorporated herein byreference. In considering these disclosures, it must be recalled,however, that unlike the cationic metallocene catalyst of the TurnerEuropean applications, the cationic metallocene catalysts employed inthe present invention must be stereorigid with dissimilar Cp rings. Thesize of the counter ion will depend on the bulk of the substituentgroups on the cyclopentadienyl rings and the manner in whichstereorigidity is imparted to the metallocene structure. Where bridgedmetallocene structures of the type disclosed in U.S. Pat. No. 4,892,851are involved, a basic requirement for production of the syndiotacticpolymers is that the cyclopentadienyl rings be dissimilar and, ofcourse, at least one ring being substituted. With stereorigidityprovided by the bridge structure, monomer insertion and isomerization iscontrolled primarily by the relationship of the anionic counterion tothe bridged structure.

Where as in this invention stereorigidity is imparted by means of directstearic hindrance between by the cyclopentadienyl substituent groupsrelatively larger substituent groups are required and stearicrelationships are developed not only between the cyclopentadienylsubstituents but also between the substituents and the noncoordinatinganion. Here, the size of the anionic counterion may be slightly smallerthan in the case of a bridged structure where stearic hindrance is notsignificant, or at least is not as significant, as in nonbridgedstructure. In addition to size, the other important characteristics ofthe anionic counterions are stability and bonding. The anion must besufficiently stable so that it cannot be rendered neutral by virtue ofthe metallocene cation extracting an anionic substituent or fragment.The bond strength with the cation is such that it must benoncoordinating or only weakly coordinating with the metallocene cationso that it makes way for the inserting monomer in the chain growingreaction.

Preferably, the syndiospecific metallocene catalysts of the presentinvention exhibit bilateral symmetry of the metallocene ligands whenviewed as planar projections of the cyclopentadienyl groups. By the term"bilateral symmetry" as used here, is meant the symmetry of the ligandas viewed through the axes of the substituted or unsubstituted Cpgroups. For example, the isopropylidene (fluorenyl) (cyclopenta-dienyl)ligand would exhibit such bilateral symmetry whereas the correspondingstructure but with the cyclopentadienyl group substituted at the threeposition would not exhibit bilateral symmetry. The corresponding ligandwith two identical substituents at the 3 and 4 position on thecyclopentadienyl group would have bilateral symmetry.

The metallocene catalysts disclosed in the Turner European applicationssuffer from certain disadvantages in that Lewis bases may be produced byprotonation of the metallocene ligand which function as poisons for themetallocene catalyst. A preferred procedure for producing cationicmetallocene catalyst of the type employed in the present inventioninvolves the reaction of an anionic compound in a noncoordinatingsolvent with a dimethyl metallocene which is unbalanced and stereorigidby virtue of a bridge between the cyclopentadienyl groups or by asterically hindered relationship between the cyclopentadienyl rings. Byway of example, triphenylcarbenium tetrakis (pentafluorophenyl) boronatemay be reacted with the neutral metallocene in a solvent such astoluene. Such catalysts and their preparation are disclosed in U.S.Patent Application Ser. No. 419,046 by John A. Ewen and Michael J. Elderfor "Preparation of Metallocene Catalysts for Polymerization of Olefins"filed on even data herewith and now abandoned the entire disclosure ofwhich in incorporated by reference.

A preferred application of the invention is in the syndiotacticpolymerization of C3+ alpha olefins, specifically propylene, but theinvention may be employed in the preparation of other polymers fromethylenically unsaturated monomers where syndiotacticity is a desiredstructure. By the term ethylenically unsaturated monomer as used hereinis meant a hydrocarbon or substituted hydrocarbon compound characterizedby a terminal vinyl group (CH2═CH--). Such compounds as may be employedin the present invention have at least three carbon atoms or are asubstituted vinyl compound, specifically vinyl chloride. They may becharacterized in terms of the following formula:

    CH2═CH--R                                              (5)

wherein: R is a hydrocarbyl group or nonhydrocarbyl substituent. Forexample, syndiospecific propagation of a polymer chain from 1-butene maybe carried out in accordance with the invention. Specific polymers inwhich syndiotacticity is sometimes desirable and to which the inventionis applicable include polyvinyl chloride and polystyrene. Thepolymerization of a 1-diene such as 1,3-butadiene may also be carriedout in accordance with the present invention to achieve a syndiotacticpolymer configuration. Syndiotactic polypropylene is probably of thegreatest practical significance and the invention will be described indetail with reference to the production of syndiotactic polypropylene.However, other compounds in which the syndiotactic configuration isdesirable are also of interest.

Polymerization procedures as disclosed in the aforementioned U.S. Pat.No. 4,892,851 may be employed in carrying out the present invention.Co-catalysts, usually organoaluminum compounds such as trialkylaluminum,trialkyloxyaluminum, dialkylaluminum halides or alkylaluminum dihalidesmay be employed in the present invention. Especially suitablealkylaluminums are trimethylaluminum and triethylaluminum with thelatter, commonly referred to as TEAL, being most preferred. However,aluminoxane which may be used as a co-catalyst in the U.S. Pat. No.4,892,851 need not be, and preferably is not, used in carrying out thepresent invention. While applicant's invention is not to be restrictedby theory, it is believed that neutral metallocenes of the typedisclosed in the parent application form cationic complexes by reactionwith the aluminoxane in the manner as disclosed by Zambelli, A. et al.,"Isotactic Polymerization of Propene: Homogenous Catalysts Based onGroup 4 Metallocenes Without Methylaluminoxane", Marco-Molecules 1989,22, pages 2186-2189. It is believed that the anionic species derivedfrom the aluminoxane compound may function to stabilize the cationicmetallocene to permit monomer insertion chain migration andisomerization during the growth of the polymer chain resulting insyndiotacity. The stereorigid cationic metallocene catalysts employed inthe present invention accomplish isomerization during monomer insertionand chain migration.

The procedures and reaction conditions disclosed in the aforementionedU.S. Pat. No. 4,892,851 may be employed in the present invention withthe exception, as noted above, that aluminoxanes need not be used andpreferably are not used. The prior art discloses the use of aluminoxanesas co-catalysts with metallocene catalysts in amounts well in excess ofa stoichiometric equivalent amount providing mole ratios of aluminum tothe coordinating metal (Me) of about 100-1000. Aluminoxanes usually arenot employed in the present invention and if they are used they are inamounts well below the aforementioned range and preferably providing anAl/Me mole rate of no more than 10 and, more preferably, no more than 1.

The catalysts used in the present invention are syndiospecific andproduce a polymer with a high syndiotactic index. As disclosed in U.S.Pat. No. 4,892,851, syndiotactic polymers generally have lower heats ofcrystallization than the corresponding isotactic polymers. In addition,for the same number of imperfections in the polymer chain, syndiotacticpolymers have a higher melting point than isotactic polymers.

The metallocene catalysts used in the present invention may becharacterized by formula (2) as described above. Me is a Group 4, 5, or6 metal from the Periodic Table of Elements but preferably is a Group 4or 5 metal and more preferably a Group 4 metal, specifically titanium,zirconium or hafnium. Vanadium is the most suitable of the Group 5metals. Each Q is a hydrocarbyl radical having 1-20 carbon atoms or is ahalogen. As a practical matter, Q will usually be a methyl or ethylgroup or a halide, preferably chlorine. In order to be syndiospecific,the Cp rings in the metallocene catalysts must be substituted in asubstantially different manner so that there is a stearic differencebetween the two Cp rings, and therefore, S'y is selected such that(CpS'y)is a substantially different substituted ring than (CpSx). Inorder to produce a syndiotactic polymer, the characteristics of thegroups substituted directly on the cyclopentadienyl rings appearimportant. Thus, by "stearic difference" or "sterically different" asused herein, it is intended to denote a difference between the steariccharacteristics of the Cp rings that controls the approach of eachsuccessive monomer unit that is added to the polymer chain. The stearicdifference between the Cp rings acts to block the approaching monomerfrom a random approach and controls the approach such that the monomeris added to the polymer chain in the syndiotactic configuration.

Without intending to limit the scope of the present invention asindicated by the claims, it is believed that in the polymerizationreaction, both the catalyst and the approaching monomer units isomerizewith each monomer addition to the polymer chain as the chain migratesbetween catalyst sites. This isomerization of the monomer which iscontrolled by the stearic blockage of the differently substituted Cprings results in the alternating configuration characteristic ofsyndiotactic polymers and is in contrast to the chain-end control of thecatalysts disclosed by Natta et al. The different reaction mechanismalso results in a different structure for the polymer.

In the preferred catalysts for use in the present invention, Me istitanium, zirconium or hafnium; Q is a hydrocarbyl group, preferablymethyl, or halogen, preferably chlorine; and k is preferably 1, but itmay vary with the valence of the metal atom. Exemplary hydrocarbylradicals in addition to methyl include ethyl, propyl, isopropyl, butyl,isobutyl, amyl, isoamyl, hexyl, heptyl, octyl, nonyl, decyl, cetyl,phenyl, and the like. Other hydrocarbyl radicals useful in the presentcatalysts include other alkyl, aryl, alkenyl, alkylaryl or arylalkylradicals. Further Sx and S'y, may comprise hydrocarbyl radicals attachedto a single carbon atom in the Cp ring as well as radicals that arebonded to two carbon atoms in the ring. The catalysts used in thepresent invention may be derived from a neutral metallocene moietyprepared in accordance with procedures such as disclosed in U.S. Pat.No. 4,892,851 which is then converted to the cationic state, followingprocedures such as disclosed in the aforementioned European applications277,003 and 277,004 or more preferably by reaction withtriphenylcarbenium boronates as described in the aforementionedcopending application Ser. No. 419,046 (now abandoned).

As noted previously for metallocene catalysts of formula (2)stereorigidity is imparted by stearic hindrance due to nonbondedinteraction between the two substituted cyclopentadienyl rings.Stereorigidity is provided due to the fact that the substituent groupsof the cyclopentadienyl rings interact in a spacial arrangement betweenrings such that rotation of the rings relative to the zirconium or othertransition metal atom is prevented or at least retarded to a substantialextent. Nonbridged neutral metallocene precursors for the catalysts offormula (2) may be prepared from substituted cyclopentadienyl radicalsin accordance with procedures described hereinafter. Examples of suchneutral metallocenes providing for direct stearic hindrance includemetallocenes of transition metals as described previously, preferablyhafnium, zirconium or titanium, in which the metallocene ligand includesring structures having two or more substituents with a total of at leastfive substituents on both cyclopentadienyl rings. Examples include(dialkyl cyclopentadienyl), (trialkylcyclopentadienyl)(trialkylcyclopentadienyl) (tetraalkylcyclopentadienyl) groups. Othersubstituted cyclopentadienyl radical pairs forming the ligand include disubstituted, tetra substituted ring pairs, tri substituted, tetrasubstituted ring pairs, and di substituted penta substituted ring pairs.Suitable ligand structures include (1,2 dialkylcyclopentadienyl) (1,3,4,trialkylcyclopentadienyl), (1,2 dialkylcyclopentadienyl) (1,3,4,trialkylcyclopentadienyl), (1,3 dialkylcyclopentadienyl) (1,3,4trialkylcyclopentadienyl), (1,2 dialkylcyclopentadienyl) (1,3,4trialkylcyclopentadienyl), (1,2,3 trialkylcyclopentadienyl),(tetralkylcyclopentadienyl), (1,2 dialkylcyclopentadienyl)(tetralkylcyclopentadienyl), (1,2,4 trialkylcyclopentadienyl),(tetralkylcyclopentadienyl), (1,3 dialkyl cyclopentadienyl) (1,2,3,4tetralkyl cyclopentadienyl), (1,3 dialkyl cyclopentadienyl) (pentalkylcyclopentadienyl). The corresponding alkylsilyl substitutedcyclopentadienyl groups may also be used in forming the metalloceneligand. Specific substituent groups which may be employed in providingdirect stearic hindrance of the metallocene catalyst include: CH3-,C2H5-,C3H7-, (CH3)3C-, (CH3)3CH2-, (CH3)3Si-, (C2H5)3C-(C2H5)3C CH2-,(C2H5)3Si-. Specific examples of sterically hindered ligand structuresinclude (1,3 dipropylcyclopentadienyl) (1,2,4 triethylcyclopentadienyl)and (1,2 diisobutylcyclopentadienyl) (triethylcyclopentadienyl). Asindicated previously, the preferred transition metals in formulating themetallocene catalysts are zirconium, hafnium and titanium. Also,condensed ring cyclopentadienyl structures can be used in providing themetallocene ligands as depicted by formula (3). Specifically, the Cpring structures may include substituted flourenyl and indenyl groups.

In the sterically hindered metallocene catalysts of the presentinvention, any suitable technique can be used to produce the substitutedcyclopentadienyl groups which may be lithiated, for example, following aprotocol such as disclosed in U.S. Pat. No. 4,892,851 for reaction witha transition metal chloride to form the neutral ligand which is thenconverted to the cationic state. However, in formulating the neutralmetallocene precursors for later conversion to the cationic metallocenesof formula (2), the lithiated substituted cyclopentadienyl groups arereacted stepwise with the transition metal salt, e.g., zirconium ortitanium tetrachloride, with the product of this reaction reacted withthe other dissimilar substituted cyclopentadienyl group. By way ofexample using the conventions Cp' and C" to designated dissimilarcyclopentadienyl groups having bulky and/or sterically hinderedsubstituents as described above, a lithiated Cp' group may be reactedwith zirconium tetrachloride to produce the dicyclopentadienyl (Cp'2)ZrCl2. The resulting product may be chlorinated to produce themonocyclopentadienyl zirconium trichloride and this product (Cp'ZrCl3)then reacted with the lithiated Cp" group to produce the product (Cp'),(Cp") ZrCl2. Those skilled in the art will recognize that this stepwisereaction formate can be followed to produce metallocene based upontitanium, hafnium, vanadium or other suitable transition metals.

Substituted cyclopentadienyl groups from which the metallocene ligandsof formula (2) are formed can be derived by any suitable technique.Suitable starting materials include benzyl alcohol, ketones ofsubstituted cyclopentadienes and substituted fulvenes. By way ofexample, a penta substituted cyclopentadiene can be produced by reactionof 5 moles of the appropriate alcohol with 1 mole of cyclopentadiene inthe presence of particulate sodium. The sodium acts to promote ringaromitization as is well known in the art.

The reaction of methyl lithium or another alkyl lithium with a tetrasubstituted dimethylfulvene may be employed to arrive at substitutedcyclopenta-dienyl group. For example, tetraalkyl-6-dimethyl-fulvene maybe reacted with methyl lithium or ethyl lithium to produce tertbutyltetraalkyl cyclopentadiene or tertamyl tetraalkyl cyclopentadiene,respectively. The resulting substituted cyclopentadienes can be reactedthrough the previously described stepwise procedure with a transitionmetal halide, e.g., titanium hafnium or zirconium tetrachloride, toproduce the corresponding dichloride in which dissimilar substitutedcyclopentadienyl groups are coordinated with the titanium, zirconium orother transition metal in the neutral metallocene complex. It willrecognized from the foregoing that numerous metallocene ligands havingbulky substituted cyclopentadienyl groups can be prepared following thereaction formats indicated above.

Another synthesis procedure involves the formation of substitutedcyclopentenones by cyclization or ring closure reactions involving oresters. The cyclization reaction can be carried out in accordance withany suitable procedure to produce the corresponding cyclopentanone. Thesubstituted cyclopentenone may be reduced to an alcohol by any of thewell known reduction reactions for conversion of cyclic ketones to thecorresponding alcohols. For example, lithium or sodium aluminum hydridecan be employed to reduce the substituted cyclopentenone to thecorresponding substituted cyclopentenol. A dehydrating agent such assulfuric acid or oxalic acid can then be used to dehydrate thesubstituted cyclopentenol to the corresponding substitutedcyclopentadiene. The reaction of polyphosphoric acid on substitutedalpha-ethylenic esters can be used for the preparation of substitutedcyclopentedienes used in formulating sterically hindered metallocenes ofFormula (2). Examples include reactions of polyphosphoric acid onsubstituted acrylates or crotonates. For example, methyl-2-n-butylcrotonate, isopropyl crotonate, or butyl crotonate can be reacted withpolyphosphoric acid to produce the corresponding substitutedcyclopentenones. Thes reactions can take place at temperatures in therange of 60°-100° C. with the reaction times varying from a few minutesto a few hours. The resulting ketones are reduced by LiAlH4 to thecorresponding alcohols and the alcohols then dehydrated to yield thedesired substituted cyclopentadienes. These, in turn, can be aromatizedand reacted with the appropriate transition metal halide, for exampletitanium, hofnium or zirconium tetrachloride to produce themetallocenes. Similar reactions with polyphosphoric acid can be carriedusing acrylic esters, e.g., methylacrylate.

The neutral metallocenes can be converted to the cationic state by anysuitable technique. Preferably, such conversion is affected using atrityl compound such as triphenylcarbenium tetrakis (pentafluorolphenylborate) as described above. Other suitable techniques are disclosed inthe aforementioned European applications 277,003 and 277,004.

Having described specific embodiments of the present invention, it willbe understood that modification thereof may be suggested to thoseskilled in the art, and it is intended to cover all such modificationsas fall within the scope of the appended claims.

I claim:
 1. A metallocene catalyst for use in the syndiotacticpropagation of a polymer chain comprising an unbalanced metallocenecation and a stable noncoordinating counter anion for said metallocenecation, said metallocene cation being characterized by cationicmetallocene ligand having sterically dissimilar ring structures joinedto a positively charged coordinating transition metal atom, each of saidring structures being a substituted cyclopentadienyl ring and one ofsaid ring structures being a substituted cyclopentadienyl group which issterically different from the other cyclopentadienyl group, and both ofsaid substituted cyclopentadienyl groups being in a sterically hinderedrelationship which each other providing a stereorigid relationshiprelative to said coordinating metal atom to prevent rotation of saidrings.
 2. The catalyst of claim 1, wherein said transition metal istitanium, zirconium, or hafnium.
 3. A metallocene catalyst for use inthe syndiotactic propagation of a polymer chain comprising an unbalancedmetallocene cation and a stable noncoordinating counter anion for saidmetallocene cation characterized by formula:

    [(CpSx)(CpS'y)MeQK].sup.+ Pa.sup.-

wherein: Cp is a cyclopentadienyl ring; each S is the same or differentand is a hydrocarbyl radical having from 1-20 carbon atoms; each S' isthe same or different and is a hydrocarbyl radical having from 1-20carbon atoms and selected such that CpSax to CpS₄ is a stericallydifferent ring than CpSay to CpS'y and is in a sterically hinderedrelationship relative to CpS'ay to CpS'y sufficient to prevent rotationof said rings and impart stereorigidity to said catalyst; Me is a Group4, 5, or 6 metal from the Periodic Table of Elements; each Q is ahydrocarhyl group having 1-20 carbon atoms or is a halogen; x is from 1to 5; y is from 1 to 5; k is from 0 to 2; P_(a) is a stablenoncoordinating counter anion.
 4. The catalyst of claim 3, wherein Me istitanium, zirconium, or hafnium and K is
 1. 5. The catalyst of claim 4,wherein Q is a halogen or a methyl or ethyl group.
 6. The catalyst ofclaim 5, wherein Q is a methyl group or chlorine.
 7. The catalyst ofclaim 3, wherein x is 2 or 3 and y is from 3-5.
 8. The catalyst of claim7, wherein S and S' are alkyl or alkylsilanyl groups containing from 1-8carbon atoms.